The OnGrid Tool assumes customer charges (i.e. flat monthly fees) and demand charges will not be offset by Solar. Those charges would always be part of the bill. So if a bill is comprised of 50% energy charges, 25% customer charges, and 25% demand charges; and if the solar system were sized to offset 100% of the energy charges, the bill offset would still be only 50%.
This effect usually only occurs for medium and large commercial/general service rate schedules.